CELL SIZE IN GINKGO AND THE PALEO-CO2 PROXY

Atmospheric levels of CO₂ are a determinant of Earth’s climate, and CO₂ levels during periods of globally warm climate, such as the late Cretaceous and Paleogene, are believed to have been higher than at present. One common method for estimating atmospheric paleo-CO₂ concentration prior to ice core records is stomatal index (SI = # of stomatal pores/# of epidermal cells per unit area). The stomatal index proxy relies on the tradeoff between minimizing water loss while ensuring that sufficient carbon is assimilated for growth and reproduction. Ginkgo has been used widely for SI analyses because Cenozoic fossil leaves, seeds, and fertile shoots assigned to G. wyomingensis are very similar to modern G. biloba.

The purpose of this study was to compare leaf epidermal cell size in G. wyomingensis and G. biloba. We used SEM images to examine the cuticle of early Eocene fossils from the Willwood Formation of the Bighorn Basin, Wyoming and modern specimens from historical herbarium collections and experimental plots. Preliminary data show limited support for a positive correlation between cell area and stomatal pore length in G. biloba, and for an increase in cell area over historical time. Both stomatal density and stomatal index are negatively correlated with cell area in G. biloba. Preliminary results also indicate that epidermal cells in G. biloba are about twice as large as those of G. wyomingensis, and this does not appear to be attributable to taphonomy. This suggests that the two species are distinct. With smaller epidermal cells and lower SI values than living Ginkgo, G. wyomingensis may have also transpired and photosynthesized at different rates. This should be taken into account in using the fossils to reconstruct paleo-CO₂ levels.